500 watts will be OK on a single timer.... keep in mind MH
lights use a capacitor to provide a large starting surge, which means they take almost 2x their rated power to start. Everything connected to that circuit will also feel that surge... whether this is a problem or not really depends on your wiring and what else is on the same circuit as the lights.
a standard 15amp household circuit is good for ~1800 watts, over the entire circuit, not at each outlet.
for example, you have 500 watts of MH
light that will take around 1000 watts to start, maybe a 200 watt heater (or two?) and another 100 in powerheads, pumps and filters ... so that would put the 'peak' load for your tank system at 1300watts, dropping down to 800 after the lights start. Now if this is a dedicated circuit, no problems, but if this circuit is also connected to lights, a tv, etc, in other places in the house, you could go over the 1800 watts breifly when the lights both start at the same time, which would trip a circuit breaker (or blown fuse)
using two timers so one light starts just a min after the other will reduce the chance you will have some sort of overload.